Oscillator with negative feedback loop



1966 R. w. BRADMILLER ETAL 3,268,834

OSCILLATOR WITH NEGATIVE FEEDBACK LOOP Filed June 10, 1964 FROM AFCCIRCUITS FIG I TERMINALS FIG 2 I I I l I I +1 l I I I l VOLTAGE I lINVENTOR. RICHARD W. BRADMILLER HAROLD F? BRUCE TIME- ATTORNEY UnitedStates Patent 3,268,834 OSCILLATOR WITH NEGATIVE FEEDBACK LOOP RichardW. Bradmiller and Harold P. Bruce, Orange County, Fla., assignors toMartin-Marietta Corporation, Middle River, Md., a corporation ofMaryland Filed June 10, 1964, Ser. No. 374,020 9 Claims. (Cl. 331113)This invention relates to frequency stable oscillators, and moreparticularly to a transistorized, frequency stable, free runningrectangular Waveform generator advantageously capable of producing ahighly stable frequency and which uniquely includes a capability ofautomatically changing the frequency of the generator by remotelyapplied DC. control voltages so as to compensate for any undesirablefrequency drifts due to any voltage or temperature variations.

In many communication and command intelligence systems, low frequenciesare utilized, and to this extent highly reliable and stable lowfrequency generating equipment are required. Generally, such lowfrequency systems utilize low frequency generators at the receivingportion of the system and usually provide an automatic frequency controlcircuit for establishing the frequency of these generators. Automaticfrequency control of the systems frequency generators is a highlydesirable and often times an essential feature for providing rapidsynchronization of the generators frequency with respect to thefrequency of the incoming signals thereby advantageously providing acapability for simultaneous or synchronous detection.

In more recently developed random access discrete address communicationsystems, such as the type described in patent application Serial No.107,194, filed May 2, 1961, in the name of McKay Goode, which isassigned to the assignee of the instant application,-there exists arequirement for a subcarn'er or clock sampling frequency in theneighborhood of 10 kc. The subcarrier or clock frequency generator ofthese systems must be highly stable in order to achieve accuratedemodulation of the incoming position modulated pulses. In this respect,it has been a common practice in some of these systems to utilize narrowband networks, such as crystal controlled, tuned L-C or tuned R-C typecircuits to gene-rate a relatively stable clock frequency.

In crystal controlled circuits, it is possible, with critically designedcircuitry, of course, to achieve stabilities of approximately 50 partsper million per degree centigrade (p.p.m./ C.), whereas in tuned L-C ortuned R-C circuits, stabilities of approximately 1000 p.p.m./ C. areattainable. In all of these prior known narrow band networks, it is mostdifficult, and in most circumstances highly impractical, to include acapability of automatically changing the frequency of the network byremotely applied control voltages (AFC). In addition, when such networksare used for sampling purposes it is necessary to include shapingamplifiers for converting the simple waveforms (e.g. sine waves)generated by the networks into complex waveforms (e.g. square waves)which are used to perform certain triggering and switching functionsinherently required in the demodulating section of todays random accessdiscrete address communication systems. An example of one recentlypatented narrow band oscillator having a reasonably stable frequencyoutput is set forth in US' Letters Patent No. 3,070,757, issued December25, 1962, in the name of A. E. Plogstedt and R. W. Bradmiller, thelatter patentee being a co-inventor of the present invention.

The prior art also teaches the use of specially designed multivibratorswhich may be either collector or emitter coupled and which can indeed bevoltage controlled for changing the frequency of the multivibrators.However, such prior known multivibrators are not capable of developing afrequency having a variation of :1% or better. Such frequency stabilityis mandatory when the generator is to be used in a random accessdiscrete address communication system.

Electronic designers are well aware of the inherent deficiencies ofprior known voltage controlled multivibrators. For example, when both afrequency stability of less than 10 p.p.m./ C., and a capability ofhandling relatively high speed commands are mutually required, such asthe case in synchronous demodulation of position modulated pulses ofaudio communication systems, multivibrators have been found unacceptablebecause of their well known quiscent instability.

The foregoing deficiencies of prior art narrow band networks and voltagecontrolled multivibrators, are uniquely eliminated by the noveltransistorized, frequency stable, free running rectangular wavegenerator of the present invention.

In accordance with the present invention a Z stage, transistorizedgenerator is utilized to generate a stabilized low frequency squarewave.This novel squarewave generator develops a relatively stable frequencywith circuit means considerably less complex than that required in priorknown multi-stage sine wave generators and shapers. Briefiy, this novelgenerator comprises in effect two al ternatively conducting activedevices, cross-connected so as to provide both a DC. forward loop and aDC. degenerative feedback loop. In addition an AC. regenerative feedbackloop is provided which includes a single reactive timing element, suchas an exponential capacitor, for discharging the emitting elements ofthe active devices, and for restoring such emitting elements to theiractive bias region, thereby alternating or switching the conductivestates of such active devices in a desired repetitious manner. That isto say, the DC. forward loop, in conjunction with the DC. regenerativefeedback loop, prevents the active devices from simultaneously being intheir high conductive states, or permit only one of such devices to bein its high conductive state during any finite interval of time;Whereas, the reactive timing element in the AC. regenerative feedbackloop restores the emitting elements of the active devices to theiractive bias region. At this instant of time during the operation of thegenerator, the AC. regenerative feedback loop drives the low conductingactive device into high conduction, and in effect momentarily overridesthe circuit function of the DC. forward loop and the DC. regenerativefeedback loop. Thus, the current conducting states of the two activedevices are alternated or reversed, or, as is commonly stated in theart, the generator switches.. As soon as this switching occurs, the DC.forward loop and DC. regenerative feedback loop again hold the activedevices in their respective conducting states until such time as thereactive element once again drives and restores the emitting elements ofthe active devices to their active bias region, thereby causing thegenerator to once again switch. In this novel squarewave generatingcircuit, it is the reactive element which primarily determines therepetition rate or frequency of the generator.

In addition to the foregoing advantageous features, the low frequencygenerator of the present invent-ion desirably permits the use of DC.control voltages in an AFC configuration, thereby automaticallyadjusting the generators frequency and dynamically compensating for anyundesirable frequency drifts of the generators frequency commonlycaused, for example, by voltage and/or temperature variations. Thisnovel generator also has a relatively wide frequency control range andconsequently is not significantly rate limited in response to externalD,C. synchronizing information, and because of these features andadvantages, it is well suited for use in the aforementionedcommunication system of McKay Goode, or in similar type communicationsystems.

It is accordingly a primary object of the present invention to provide atransistorized, frequency stable, free running rectangular waveformgenerator.

Another object of the present invention is to provide a waveformgenerator of the type described which further includes a capability ofautomatically changing the frequency of the generator by remotelyapplied D.C. control voltages. 7

Another object of the present invention is to provide a waveformgenerator of the type described which is advantageously capable ofdeveloping a highly stable clock sampling frequency for use as the clockgenerator in a random access discrete address communicaton system.

Another object of the present invention is to provide a squarewavegenerator of the type described which utilizes both a DC. forward loopand a DC. degenerative feedback loop in circuit combination with an AC.regenerative feedback loop having a single reactive timing element,whereby a DC control voltage in an AFC configuration can beadvantageously utilized to dynamically compensate for undesirablefrequency drifts caused, for example, by voltage and/or temperaturevariations.

Another object of the present invention is to provide a squarewavegenerator of the type described which has a relatively wide frequencycontrol range and is not significantly rate limited in response to DC.synchronizing information. These and further objects and advantages ofthe present invention will become more apparent upon reference to thefollowing and claims and the appended drawings wherein:

FIGURE 1 is a circuit diagram of a preferred enrbodiment of thesquarewave generator in accordance with the present invention, with theDC. automatic frequency control (AFC) voltages developed by conventionalAFC circuits (not shown) being applied to terminal A, and the outputbeing developed across resistor R FIGURE 2 depicts waveforms present atseveral app-ropriate terminals in the circuit of FIG. 1, with thevertical dashed lines, which represent pertinent time periods, includedto assist in the detailed explanation of the circuit of FIG. 1 and itsmode of operation.

Referring specifically to FIG. 1, a preferred embodiment of thefrequency stable oscillator is shown as comprising two transistors T andT each having emitter, collector and base electrodes. Transistor T hasits emitter connected to source +V via terminal A and variable resistorR while its collector is connected to ground via terminal B and variableresistor R whereas, transistor T has its collector connected to groundvia ter.- minal C and variable resistor R while its emitter is connectedto source +V via terminal D, variable resistor R terminal E, andvariable resistor R Note here, that the value of resistors R and R inthe emittercollector circuit of transistor T and the value of resistorsR -R in the emitter-collector circuit of transistor T and thecross-connection of the bases of transistors T and T to the resistivebias strings in the emitter circuits of transistors T and Trespectively, determines the stabilized operating bias for transistors Tand T Base bias for transistor T is provided by means of a directconnection between terminal E and the base of transistor T whereas basebias for transistor T is provided by means of a direct connectionbetween terminal B and the base of transistor T As will therefore beseen, an oscillator is formed wherein the repetition rate or frequencythereof is provided by the capacitor C which is directly connectedbetween terminals A and D. The rectangular waveform generated by thefrequency stable oscillator is derived across resistor R which isconnected between terminal C and ground, and such output appears betweenterminal F and G.

In order to set up the oscillator of FIG. 1 for stable low frequencyoscillation, the capacitor C is effectively disconnected from circuitand the variable resistors R to R are adjusted for stable direct currentoperation. The emitter resistor R in parallel with the seriescombination of resistors R and the reflected emitter impedance oftransistor T with respect to source +V, constitutes a base drivingimpedance for transistor T Accordingly, by maintaining the base drivingimpedance low in comparison to the base input impedance of transistor Tstable direct current operation results over a wide variation intransistor parameters.

When capacitor C is effectively connected back into the circuit, thecross-coupled transistorized circuit of FIG. 1 goes into oscillation ata frequency directly determined by the values of variable resistors R toR and by the value of capacitor C thereby deriving a stable lowfrequency output.

It should be noted at this point that the direct connection betweenterminal B and the base of transistor T constitutes a forward loop; thatthe direct connection between terminal E to the base of transistor Tconstitutes a degenerative or negative feedback loop; and that thecapacitor C connected between terminals A and D, which in effect is areactive coupling between the emitters of transistors T and Tconstitutes a regenerative or positive feedback loop.

The forward loop, which comprises terminal B, basecollecto-r path oftransistor T terminal C, resistor R ground and resistor R provides thebase bias for transistor T The negative feedback loop, which comprisesterminal B, base-emitter path of transistor T terminal D, resistor Rterminal E, and the base-collector path of transistor T provides thebase bias for transistor T and insures that the oscillator of FIG. 1will be free running. In addition, the negative feedback loop insuresthat the forward gain of the oscillator is equal to and that suchoscillator is relatively independent of transistor parameters. Thepositive feedback loop, which comprises terminal A, emitter-collectorpath of transistor T terminal B, basecmitter path of transistor Tterminal D and capacitor C insures that the oscillator of FIG. 1 willoscillate, i.e., transistors T and T alternatively switch from theirhigh to their low conductivity states. This is so because the overallgain of the oscillator is equal to or greater than one for allfrequencies where X,, which is the impedance of capacitor C is equal toor greater than the impedance of the emitter of transistor T Thisactually determines the repetition rate of the oscillator. Note herethat the charge and discharge paths for capacitor C during the timeintervals t -t t -t t -t etc. as shown in FIG. 2, involve all of theresistors R R as well as the saturated current gain and leakage of bothtransistors T and T Note at this point that a DC. control voltage iscoupled to terminal A. That is to say, the novel oscillator describedherein, unlike prior known stable oscillators, is uniquely capable ofbeing frequency controlled by a DC. control voltage conventionallydeveloped by well known AFC circuits. Thus, by comparing the AG.component of the squarewave appearing across terminals F and G with astandard AC. voltage, a DC. error voltage can be developed via wellknown AFC circuitry. This D.C. error voltage when applied to theoscillator via terminal A, for example, advantageously and rapidlycompensates for any frequency shifts of the output squarewave from adesired and predetermined frequency.

Accordingly, when the DO error or control voltage is applied on terminalA, a new operating frequency is automatically established. What is mostimportant at this point is the fact that this new operating frequency isestablished without destroying the aforementioned stabilizedcharacteristics, thus achieving desirable long term stability forinternal variations of temperature and component tolerances while yetstill exhibiting a capability of rapid control of the oscillatorsfrequency by automatic externally developed DC. control voltages.

Note here that the capacitor C which is connected in a regenerativefeedback loop arrangement, does not adversely affect the response timeof the oscillator, as is the case in oscillators utilizing narrow bandselective networks. This desirable response time characteristic of theoscillator of the present invention is possible because of the wide bandwidth characteristics of the reactive network of the regenerativefeedback loop.

Although the resistors R through R are shown as variable resistors forpurposes of explaining the balanced operation of the oscillator of thepresent invention, fixed resistors are normally utilized, and theinterdependence thereof must be established to provide this balancedoperation as will be discussed in detail below. The operation of theoscillator of FIG. 1 in light of the waveforms of FIG. 2 follows.

At time t let it be assumed that the voltages at terminals A-E are shownin waveforms 11 to 19, respectively, of FIG. 2. Note that during timeinterval t -t the voltage of waveforms 17 and 19 are slowly increasing.

During time interval I 4 the capacitor C in the regenerative feedbackloop between the terminals A and D commences to charge thereby slowlyallowing the voltage on terminal D to increase as shown in waveform 17.When voltage on terminal D exceeds the voltage on terminal B, which isin effect the bias voltage for the base of transistor T a switchingaction takes place. That is to say, transistor T is rapidly driven intoa heavy conducting state which rapidly causes the voltages at bothterminals D and B to fall. This rapid decrease of the voltages ofwaveforms 17 and 11 occur at time t Note here that the voltages atterminals A and E, i.e., waveforms 11 and 19, also rapidly decrease,when Waveform 17 exceeds waveform 13, whereas, the voltage at terminalC, i.e., waveform 15, rapidly increases at the occurance of this event.This is due to the switching of transistor T from a low conductive stateto a high conductive state.

Attime 1 when the voltage on terminal D, i.e., emitter of transistor Texceeds the voltage on terminal B, i.e., base of transistor T transistorT is driven rapidly into heavy conduction, and the voltages at terminalsD and B rapidly fall or in effect such voltage terminals follow thevoltage change on terminal A, which is the emitter of transistor T Notehere that the voltage at terminal B, which follows the voltage atterminal A causes the transistor T to conduct heavily, thereby drivingthe voltage at terminal D to even a greater negative value. Due to thecapacitor action of C transistor T is driven into a low conductivitystate. Although transistor T experiences a reduced current conduction,the voltage on terminal A is being driven through the capacitor C andfollows the voltage change on terminal D.

Let us assumenow that time t represents the beginning of a completecycle of operation of the novel oscillator of the present invention.During the time interval 1 4 due to the degenerative feedback loopbetween terminals E and the base of transistor E the voltage at terminalA rises slowly to a voltage level exceeding the voltage level atterminal E (note waveforms 11 and 19), which in effect exceeds the biasestablished at the base of transistor T When the voltage at terminal A,i.e., emitter of transistor T exceeds the voltage level at terminal E,i.e., base of transistor T a switching action again occurs, and thetransistor T is now rapidly driven into its high conductivity state.Note that the voltage on terminal A is again driven through thecapacitor C and caused to follow the voltage change on terminal D. Whenthe switching action occurs, transistor T is rapidly driven into its lowconductivity state, which in turn rapidly causes the voltage atterminals D to rise, toward V+ and the voltage at C to fall towardground. Note that the change in voltage at terminal D, through capacitoraction of C drives terminal A and that terminal B, because it is in theemitter-collector circuit path of transistor T follows the voltagechange on terminal A. This rapid increase of voltages of waveforms 17and 11 occur at time 1 Note also, that the voltages at terminals A and Ealso rapidly rise. This is due to the switching of transistor T from itshigh conductivity state.

At time t when the voltage on terminal A, i.e., emitter of transistor Texceeds the voltage on terminal E, i.e., base of transistor T transistorT is driven rapidly into its high conductivity state, and the transistorT is driven rapidly into its low conductivity state. Thus, the voltagesat terminals D, 13, A and B rapidly rise, as shown in FIG. -2 at thistime period.

Let us now assume that time t represents the end of the first half cycleof operation of the novel oscillator of FIG. 1.

During the time interval 23 -1 the slow changing circuit conditionsabove described with regard to time interval t t are repeated. Thus, attime t the oscillator again switches as explained above regarding theswitching action occurring at time t Let us now assume that time t;,represents the end of the full cycle of operation. Thus, the waveforms11-19 during time interval t -t represents one complete cycle ofoperation of the oscillator of FIG. 1. Note here that the voltagespresent on terminals A-E during each of the times t t and t reversepolarity in a considerably small interval of time which for purposes ofthis discussion may be considered substantially simultaneous.

The waveform at terminal C, or output terminals F and G, is a squarewavehaving rapid rise and fall times. Of course, output waveform 15 maybe'coupled to a positive and negative clamp for providing any desiredvoltage peaks.

Note at this point that the switching feature of the oscillator of thepresent invention, which is due to the novel cross coupling arrangementof transistors T and T provides a repetitious squarewave, and that suchoscillator is free running. Note further, that the charge and dischargepath for capacitor C during each halfcycle involves each of theresistors R -R as well as the saturated current gain of each of thetransistors T and T It has been established through extensiveexperimentation that the frequency stability of the oscillator of FIG. 1is considerably high. For exemplary purposes the below chart showson-time variations of the transistors T and T when any one of theresistors R R experiences a 10% decrease in value.

On Time (Percent Percent Percent As can be seen from the above chart fora given active device, the oscillator of the present invention uniquelyprovides a capability of balancing or compensating for voltage andtemperature variations so as to advantageous- 1y provide several ordersof magnitude improvement in frequency stability. The circuit of FIG. 1provides frequency stability improvement from 5000 p.p.m./ C. to betterthan 200 p.p.m./ C. It has also been established 7 that stabilities of 2p.p.m./ C. can be readily achieved by incorporating a 3.2K resistor anda 1.5K sensitor for resistor R For exemplary purposes only the followingvalues for the components of the circuit of FIG. 1 are included asfollows:

T1 and T2 C .068 microfarad. R 820 ohms.

R 4.7 kilohms.

R 680 ohms.

R 470 ohms.

R 220 ohms.

+V source 12 volts D C.

The terms and expressions which have been employed herein are used asterms of description and not of limitation and it is not intended, inthe use of such terms and expressions, to exclude any equivalents of thefeatures shown and described, or portions thereof, but it is recognizedthat various modifications are possible within the scope of the presentinvention.

Without further elaboration, the foregoing is considered to explain thecharacter of the present invention so that others may, by applyingcurrent knowledge, readily adapt the same for use under varyingconditions of service while still retaining certain features which mayproperly be said to constitute the essential items of novelty involved,which items are intended to be defined and secured by the appendedclaims.

We claim:

1. An oscillator for generating a stable, low frequency, substantiallyrectangular wave comprising, in combination:

(a) first and second active means, each including electron emitting,collecting and control means;

(b) means for coupling the collecting means of said first active meansto the control means of said second active means in a forward loopmanner;

(c) means for coupling the emitting means of said second active means tothe control means of said first active means in a degenerative manner;and

(d) reactive means for coupling together said emitting means of saidfirst and second active means in a regenerative manner, whereby saidreactive means establishes the repetition rate of said oscillator.

2. An oscillator in accordance with claim 1, wherein:

(a) an external control voltage is coupled to said oscillator forautomatically adjusting the repetition rate of said oscillator.

3. An oscillator in accordance with claim 2, wherein:

(a) said active means are solid state devices having emitter collectorand control electrodes; and

(b) said reactive means is a capacitor.

4. A free running oscillator for generating a stable, low frequency,substantially rectangular wave comprising, in combination:

(a) first and second active means, each including electron emitting,collecting and control means;

(b) a source of potential;

(c) resistive means for respectively coupling the collecting means ofeach of said active means to ground;

((1) resistive means for respectively coupling the emitting means ofeach of said means to said source of potential;

(e) D.C. means for coupling the collecting means of said first activemeans to the control means of said second active means in a forward loopcircuit arrangement;

(f) D.C. means for coupling the emitting means of said second activemeans to the control means of said first active means in a degenerativecircuit arrangement; and

(g) reactive means for coupling together said emitting means of saidfirst and second active means in a regenerative circuit arrangement,whereby said reactive means establishes the frequency of saidoscillator.

5. An oscillator in accordance with claim 4, wherein:

(a) said active means are solid state devices having emitter, collectorand control electrodes;

(b) said resistive means are resistors; and

(c) said reactive means is a capacitor.

6. An oscillator in accordance with claim 5, wherein:

(a) an external D.C. control voltage is coupled to said first solidstate device for automatically adjusting the frequency of saidoscillator.

7. A free running oscillator for generating a stable, low frequency,substantially square wave comprising, in combination:

(a) first and second active devices, each including current emitting,current collecting and current contro means;

(-b) a D.C. source of potential;

(c) said collecting means of each of said active devices being connectedto ground through respective first and second resistive means;

(d) said emitting means of said first active device being connected tosaid source through a third resistive means;

(c) said emit-ting means of said second active device being connected tosaid source through a fourth and fifth series connected resistive means;

(f) said control means of said first active device being connected tothe junction of said fourth and fifth resistive means so as to provide adegenerative feedback loop;

(g) said control means of said second active device being connected tosaid collecting means of said first active device so as to provide aforward loop; and

(h) reactive means coupled between said emitting means of said first andsecond active devices so as to provide a regenerative feedback loop,whereby said rcactive means establishes the frequency of saidoscillator.

8. An oscillator in accordance with claim 7, wherein:

(-a) said active devices are transistors having emitter,

collector and control electrodes;

(b) said resistive means are resistors; and

(c) said reactive means is a capacitor.

9. An oscillator in accordance with claim 8, wherein:

(a) an external D.C. control voltage is coupled to said emitterelectrode of said first transistor for automatically adjusting thefrequency of said oscillator.

No references cited.

ROY LAKE, Primary Examiner.

J KOMINSKI, Assistant Examiner.

1. AN OSCILLATOR FOR GENERATING A STABLE, LOW FREQUENCY, SUBSTANTIALLYRECTANGULAR WAVE COMPRISING, IN COMBINATION: (A) FIRST AND SECOND ACTIVEMEANS, EACH INCLUDING ELECTRON EMITTING, COLLECTING AND CONTROL MEANS;(B) MEANS FOR COUPLING THE COLLECTION MEANS OF SAID FIRST ACTIVE MEANSTO THE CONTROL MEANS OF SAID SECOND ACTIVE MEANS IN A FORWARD LOOPMANNER; (C) MEANS FOR COUPLING THE EMITTING MEANS OF SAID SECOND ACTIVEMEANS TO THE CONTROL MEANS OF SAID FIRST ACTIVE MEANS IN A DEGENERATIVEMANNER; AND (D) REACTIVE MEANS FOR COUPLING TOGETHER SAID EMITTING MEANSOF SAID FIRST AND SECOND ACTIVE MEANS IN A REGENERATIVE MANNER, WHEREBYSAID REACTIVE MEANS ESTABLISHES THE REPETITION RATE OF SAID OSCILLATOR.